1. Field of the Invention
The invention relates generally to projection exposure apparatus and projections lenses used therein. Such apparatus are used for the microlithographic manufacture of electronic circuits and other microstructured devices.
2. Description of Related Art
U.S. patent application Ser. No. 10/177,580 discloses purely refractive and catadioptric projection lenses, with numerical apertures of 0.8 and 0.9 at an operating wavelength of 157 nm, which are suitable for projection exposure apparatus known from the market.
For projection wavelengths in the deep ultraviolet (DUV) fluoride crystals are used as material for lenses and other refractive optical elements. U.S. Pat. No. 6,201,634 B describes that technical fluoride crystals suitable for this use have stress birefringence which exhibits direction dependency with respect to the crystal axes.
It is known from the Internet publication “Preliminary Determination of an Intrinsic Birefringence in CaF2” by John H. Burnett, Eric L. Shirley, and Zachary H. Lewin, NIST Gaithersburg Md. 20899 USA (released on Jul. 5, 2001) that, in addition to stress-induced birefringence, calcium fluoride single crystals also exhibit intrinsic birefringence.
All these cited documents are also intended to be part of the disclosure of this application in their full scope.
These birefringent effects are significant only at short wavelengths below about 200 nm, i.e. in particular at 193 nm and to a greater extent at 157 nm, the wavelengths which are preferred for high-resolution microlithography.
Since this birefringence is dependent on the light-ray direction with respect to the crystal axes, a variation is encountered as a function of both the acceptance angle and the rotation angle (azimuth angle) about the optical axis.
For an optical element, in particular a lens (although it may also be designed as a plane plate, for example a terminating plate or a filter) which is oriented rotationally symmetrically about the (111) crystal axis, the birefringence is a minimum for normal transmission of the light ray. With an acceptance angle of about 35° and at three rotation angles (azimuth angles) mutually rotated by 120°, however, the incidence direction is equivalent to the (110) orientation of the crystal and maximum birefringence occurs.
With a rotationally symmetrical arrangement relative to one of the (100), (010) or (001) axes, the (110)-equivalent axes with maximum birefringence are then found with fourfold rotational symmetry for an acceptance angle of 45°.
Now, with an element made of CaF2 from which a 157 nm light ray emerges with the numerical aperture 0.8, the acceptance angle for transmission with the refractive index of about 1.56 is equal to 31°; for NA=0.9, an angle of about 35° is found. The direction-dependent birefringence is therefore a problem with such wide-acceptance systems.